Dylan,
I'd feel bad if the show's writer ever came out and said he was a ManUre fan. Kind of like the Tranmere fans who think Elvis Costello is one of them, only to discover him shopping at the RS superstore.
Onslow is a favourite character of mine. Behind the slob there is a great degree of intelligence and likability. He's comfortable amongst the rich or the working class and stays himself regardless.
I remember that episode as well. It stuck with me even though I don't recall Onslow ever proclaiming an allegiance to a club.